1. Field of the Invention
The teachings herein relate to a detector of ionizing radiation and more particularly to a detector for detecting a gamma component and a neutron component.
2. Description of the Related Art
Detection of radioactive materials, particularly those illicitly hidden in the stream of commerce, requires the availability of a variety of radiation detection equipment. In particular, Hand-Held RadioIsotope Identification Devices (HHRIID) are needed in the field to quickly determine the presence of special nuclear material and distinguish it from the presence of medical and industrial radioisotopes, as well as from normally occurring radioactive material. One possible embodiment of an HHRIID consists of two optically separated radiation sensors that emit light and are coupled to a common photodetector. The first radiation sensor is a neutron sensing component that contains atomic nuclei with a high neutron cross section, such as 6Li in a chemical compound, such as 6LiF, surrounded by particles of a scintillator material, for example, ZnS:Ag, and bound together in an epoxy matrix. The second radiation sensor is a gamma sensing component and consists of a scintillator crystal with enhanced gamma energy resolution, high gamma stopping power, and an atomic composition with very low neutron absorption cross section. The two radiation sensors are optically separated in such a manner that the light emitted by one sensor does not reach the other sensor in order to avoid optical crosstalk. The HHRIID may include a pulse shape discrimination circuit that identifies the source of light emitted (either by the neutron sensing component or the gamma sensing component based on the difference in scintillation light decay times.)
In the detection of neutrons via solid-state scintillation, perhaps the most highly-utilized material stems from a granular mixture of 6LiF and ZnS:Ag. Each component in this mixture represents “best-of-class” performance (i.e., respectively, neutron capture and luminescence). For neutron capture, the LiF crystal structure offers one of the highest Li atom densities in solid-state form and maximizes the probability of neutron interaction, especially if in addition it is enriched in 6Li. Furthermore, the absorption of thermal neutrons by 6Li induces direct disintegration into alpha and triton particles with no other secondary radiation. The absence of multiple reaction pathways and/or radiation by-products enables one to optimize the corresponding phosphor to a single secondary radiation type (i.e., heavy charged particles). For luminescence, ZnS:Ag is one of the brightest phosphors known and remains unparalleled in its emission under alpha and triton exposure.
A crucial metric in determining the performance of a neutron scintillator is neutron sensitivity, the number of neutron events registered per incoming neutron flux. This measurement requires the collection and counting of photons from the neutron scintillator. However, the light output of 6LiF/ZnS:Ag materials is limited by two factors: [1] self-absorption of the emitted light by the ZnS:Ag phosphor, and [2] optical attenuation of the emission photons via scattering. The latter arises due to the granular nature of the material (i.e., the multitude of interfaces with index-of-refraction mismatches). The end result is a threshold in thickness beyond which further (useful) light output is unachievable.
Conventional neutron detection approaches typically rely on the optical coupling of a thin disk of 6LiF/ZnS:Ag composite material (<1 mm) to the flat, circular face of a photosensor. For reasons stated above, the neutron sensitivity of this design cannot be improved by increasing the thickness of the disk. Instead, multiple layers of 6LiF/ZnS:Ag composite material must be employed, which in turn, create substantial difficulties in transporting the resulting additional light to the photosensor(s). Furthermore, a flat disk may not be the desired shape for neutron capture. If an application requires the moderation of neutron energies (i.e., reduction to ambient, thermal energies), cylindrical shells are preferable to disks. For this geometry, the challenge of light transport becomes even more acute.
In order to improve the total neutron sensitivity of the detector while providing a design that reduces both the size and weight of the detector, an optimal integrated gamma/neutron detector must address the issue of packaging a larger area of neutron sensing composite material.
In addition, neutron scintillating composite (NSC) materials, when comprised of granular mixtures (e.g., 6LiF/ZnS:Ag, 10B2O3/ZnS:Ag, etc.), suffer from optical losses due to the internal scattering and absorption of light. The commercial utility of these mixtures, however, remains high due to their exceptionally low cost. In addition, ZnS:Ag is known to self-absorb a fraction of its own emission. Despite this loss mechanism, ZnS:Ag remains a useful phosphor because its energy conversion is the highest known for alpha and triton particles. ZnS:Ag is also inexpensive, having been produced commercially for decades as a standard blue phosphor for CRTs.
These two loss mechanisms, separately or combined, produce a thickness limitation: increasing the NSC thickness beyond a certain threshold value—1.0 mm for 6LiF/ZnS:Ag mixtures—provides no additional light output despite the additional capability for neutron absorption.